John Reid admits today that the Home Office will continue to be undermined by new crises and embarrassments before he is able to bring the department under control.

After another weekend of woe for the home secretary - including claims that ministers did nothing to tighten a loophole allowing sex offenders to escape monitoring in the community - Mr Reid concedes he is contending with both inherited and new failings discovered by his reforms, and others found by the media.

"If you renovate a house you start by taking the wallpaper off. It is only then that you discover more problems. That's what it is like in the Home Office," he writes in an article in the Guardian.

"These problems don't leave me beleaguered. If we weren't discovering more we wouldn't be reforming the Home Office. Indeed I expect more problems." He also makes clear he has no intention to quit.

"I was sent to the Home Office to do a job. Being home secretary is my biggest challenge. But it isn't mission impossible. Judge me not on the challenges but on my response to them.

Defending his record from media critics - and allies of Gordon Brown who want him sacked if the chancellor becomes prime minister - Mr Reid says: "Some people see their main task as being to change ministers at the Home Office; I see my main task as changing the Home Office."

Mr Reid's defiance comes after a weekend of further disclosures and recriminations from his political opponents.

It emerged that the Identity and Passport service had failed to enforce travel bans on 147 convicted drug traffickers. And after inquiries to all 50 police forces in England and Wales, the News of the World claimed that they had lost track of a total of 322 sex offenders.

The Police Federation said they had warned three years ago about a loophole allowing offenders to give vague addresses. The Tories said they had raised the issue last October. "If John Reid was doing his job he would have got a grip on the issue then," said David Davis, shadow home secretary. "His staggering complacency speaks volumes about his lack of focus on issues that concern public safety ."

The Tories also mocked the Home Office's claim that it was considering using compulsory lie detector tests on sex offenders after their release, saying this had been announced three times.

The former Conservative home secretary Kenneth Clarke added: "Reid is almost beyond saving. It's a long time since I remember a home secretary ... that has got himself in such quite deep water than John Reid is this weekend."

Mr Reid is braced for further criticism this week when Anne Owers, the chief inspector of prisons, uses her annual report to highlight the rise in the number of prisoners serving indeterminate sentences to about 2,000. And in another potentially awkward intervention, Rowan Williams, the Archbishop of Canterbury, will deliver a speech to the Prison Reform Trust on Thursday.

Rehearsing his defence against Ms Owers, the home secretary writes today that the rise in indeterminate sentences for the most dangerous offenders was balanced in the 2003 criminal justice act by a 15% reduction in sentence length.

Tony Blair also defended the government's approach. He told the BBC: "Some of these things like foreign prisoners, or these offences that have been committed abroad by British people who then return back home, the reason we're dealing with these now is that for the first time there is a system in place to deal with them."